Pre-wetting is the process of for example spraying deicing salt with a solution of liquid chemical before spreading the salt on a roadway. Pre-wetting the salt helps it work more effectively as a deicing agent for two reasons: Firsts wet salt clings to the road instead of bouncing off or being swept off by traffic. The result is that less salt is spread, saving money and minimizing the threat to the environment. Second, to be effective as a deicing agent, salt requires moisture. Moisture dissolves the salt, releasing heat and thereby melting the ice and snow, as well as breaking the ice-road bond. When temperatures drop below freezing there is no moisture on the road, and salt alone is ineffective. Pre-wetting the salt ensures that there will be enough moisture to facilitate the melting process. There are several chemicals used to pre-wet salt. The most inexpensive is the use of sodium chloride and water or salt brine which is a salt and water solution. See Semisequicentennial Transportation Conference Proceedings, May 1996, Iowa State University.
Since brine is what actually melts snow and ice, solid rock salt must first create brine before it goes to work. In the absence of heat, sunlight, friction, or in low available moisture levels, solid salt is slower to go to brine and then to work. Road salt (sodium chloride) has an effective temperature range above 20-25 degrees Fahrenheit. When temperatures drop to around 10 degrees F. or below, road salt has significantly reduced melting capacity. The addition of a liquid chemical to salt enhances its ability to provide safe levels of service by increasing the speed at which salt creates brine to melt snow or ice. Greater chloride efficiency is achieved when liquid chemicals are added to salt by lowering the “effective” temperature range of the salt.
There are many methods in place to pre-wet salt. Some agencies have sprayed liquid chemicals over loads in the spreaders creating a “hot load”. This method does not uniformly coat all the salt in the spreader and often runs out the back. Some liquid chemicals can “fuse” the salt in the spreader if not emptied, thereby adding to maintenance headaches and clumps in the salt. Others apply a given amount to each loader bucket prior to loading in the spreader. This method is somewhat more effective than direct over the top applications. See Mark Cornwell, Ice Control Engineering, Syntech Products Corporation.
Another method of wetting the salt is to spray the salt with sodium chloride or salt brine as it is leaving the spreader. Nozzles are mounted adjacent the spinner and spray the salt as it is being spread onto the surface. The problem with pre-wetting in this way is over spraying of the chemical which is uneconomical and provides an uneven application of the pre-wetting solution to the salt which results in the salt bouncing off the surface and the salt not being effective to melt the snow and ice. Additionally, there is the problem of evaporation and drifting of product which are corrosive when put into the air.